Command and Data Handling (C&DH)

1. Command and Data Handling(C&DH) AERSP 401A

2. C&DH • Principal Function • Processes and distributes commands; processes, stores, and formats data • Other Names • Spacecraft Computer System • Spacecraft Processor • References • 10.4.4, 11.3, Chapter 16

3. Sizing the C&DH Subsystem

4. Typical Characteristics of Basic Components for C&DH

5. Space Computer System Key Concerns • More on-board processing is desirable and inevitable • Can increase functionality with major savings in weight, power, and cost • Can significantly reduce operations and, therefore, lifecycle costs • Can dramatically increase system flexibility – software is, but far, the least expensive “on-orbit replacement unit” • Formally correct approach is to decide on the appropriate software and then size the computer to meet the need • Not a realistic alternative in today’s environment for most functions • Net result is a major performance, cost, and schedule risk • Key computer system trades • On-board vs. ground processing • What to automate • Selection and sizing of hardware • Language selection • Software selection or development

6. Computer Systems Development Process

7. Earth Sensor Thrusters Central Processor GPS Accelerometer Star Tracker Wheels Gyros Candidate Architectures Centralized Architecture

8. Earth Sensor Thrusters Central Processor GPS Accelerometer Star Tracker Gyros Wheels Candidate Architectures Ring Architecture

9. Earth Sensor Thrusters Central Processor GPS Accelerometer Star Tracker Wheels Candidate Architectures BUS Architecture Gyros

10. Architecture Trades • All architectures must meet mission requirements to be considered in architecture trades • Typically, architecture trades are somewhat subjective, although standard discriminators exist: Final processing architecture is frequently a hybrid – optimizing all aspects of the design

11. Architecture Selection • If mission requirements strongly emphasize weight and power and those budgets are fixed, some reliability and fault tolerance must be sacrificed to meet needs. • If mission life is critical and payload operation is the highest requirement, some additional weight and power may be needed to enhance reliability.

12. Summary • Software offers major advantages in performance, recurring cost, “lifecycle”, flexibility, and reliability • High autonomy offset by significant disadvantages • High non-recurring cost • Development risk is large – only way to know the requirements is to write the software in advance • Difficult process to manage and measure progress • In the near term, computers will become most important in: • LightSats where needed performance cannot be achieved without them • Constellations or recurring applications, where non-recurring cost can be amortized

13. Command Decoder Block Diagram Command Outputs Pulse Pulse Data Clock Enable High-Level Discrete Uplink Command Message Decoding Command Source Arbitration Command Message Validation Onboard Computer Low-Level Discrete Hardline Test Serial Digital Over/Under Voltage Detect Command Decoder Prime Power

14. Data Handling Unit Block Diagram Multiplexed Signal Inputs High-Level Analog Analog to Digital Converter Data Formatter and Control Logic Downlink Low-Level Analog Onboard Computer Passive Analog Hardline Test Bi-Level Serial Digital